1. Field of the Invention
The present invention generally relates to nuclear medicine, and systems for obtaining nuclear medical images of a patient's body organs of interest. In particular, the present invention relates to a collimator for an array of radioactive lines for use in nuclear medicine imaging, particularly for single photon imaging including single photon emission computed tomography (SPECT), and particularly adapted for use on a SPECT system for non-uniform attenuation correction measurements.
2. Description of the Background Art
Nuclear medicine is a unique medical specialty wherein radiation is used to acquire images that show the function and anatomy of organs, bones or tissues of the body. Radiopharmaceuticals are introduced into the body, either by injection or ingestion, and are attracted to specific organs, bones or tissues of interest. Such radiopharmaceuticals produce gamma photon emissions that emanate from the body. One or more detectors are used to detect the emitted gamma photons, and the information collected from the detector(s) is processed to calculate the position of origin of the emitted photon from the source (i.e., the body organ or tissue under study). The accumulation of a large number of emitted gamma positions allows an image of the organ or tissue under study to be displayed.
In a conventional SPECT (Single Photon Emission Computed Tomography) study of an organ such as the heart, a radioisotope (Tc-99m, T1-201, for example) is administered to the patient and the radioisotope is taken up by the heart muscles. Then, the patient is placed in a scintillation camera system and one or more scintillation camera detectors are rotated about the long axis of the patient. These detectors pick up gamma radiation that leaves the patient, and the resulting data is used to form three-dimensional images (“SPECT images” or “tomographic images”) of the distribution of the radioisotope within the patient.
Such three dimensional SPECT images can be calculated based on a set of two-dimensional images (“projections” or “projection images”) acquired by the scintillation camera system; this calculation process is known as image reconstruction. The most commonly employed method of image reconstruction is known as filtered backprojection (FBP). When FBP reconstruction is used to reconstruct SPECT images from scintigraphic projection images obtained from a scintillation camera, some well-known distortions introduce errors (“artifacts”) in the result. One of the most important distortions is caused by attenuation of gamma radiation in tissue.
As a consequence of attenuation, image values in the various projections do not represent line integrals of the radioisotope distribution within the body. It is therefore necessary to correct for this, and the process for doing so in SPECT is known as attenuation correction.
Many techniques for attenuation correction in SPECT assume that the linear attenuation coefficient of the body is uniform and impose such uniformity as a mathematical constraint in the image reconstruction process. However, for a very important class of studies, namely cardiac SPECT studies, the linear attenuation coefficient of the body is in fact highly nonuniform. This is because lung tissue has a lower attenuation than do, e.g., the blood and other non-lung tissue. Thus, in SPECT studies of, e.g., the heart, a SPECT reconstruction of the image of radioactivity within the heart will necessarily contain artifacts caused by the unequal attenuation coefficients of, e.g., the lungs and the body. Such artifacts also appear in SPECT cardiac images taken from obese patients and from large-breasted female patients.
It is known to measure the actual attenuation coefficients of body tissues by placing a line source of gamma radiation on one side of the body and measuring the transmission of the gamma radiation through the body as a function of direction, i.e. collecting transmission CT data, as the line source is scanned across the patient's body. See, for example, U.S. Pat. No. 5,576,545 which describes a system for correcting attenuation artifacts in a SPECT study in which a line source is parallel to the axis of rotation of the scintillation camera detector(s). The line source is scanned in a plane that is parallel to the detector(s).
U.S. Pat. No. 5,650,625 describes an improvement to the system described in the '545 patent. The improvement comprises a two-dimensional radiation emitter. The radiation emitter is rectangular in shape and has an array formed of a plurality of parallel, elongated line sources of equal length supported parallel to the axis of rotation of the camera system by a support. In one embodiment, the line sources have different radiation densities with the maximum density in the central region of the emitter, although the line sources with maximum density do not need to be centered with respect to the emitter. The line sources can be moved between different predetermined locations in the support.
It is also known to use collimators with the line source. These collimators are known as source collimators. For example, U.S. Pat. No. 6,060,712 describes the use of collimator with the source radiation to limit the radiation that passes towards the patient to the radiation that is substantially parallel to the collimator. U.S. Pat. No. 6,271,524 describes a source collimator having a plurality of apertures preferably distributed in a plurality of rows. A cartridge containing the source, attenuator, shielding and collimator insert has been developed for use in a line source array. (Bak, D. J., “Shift & Replenish,” Design News, Apr. 1, 2004.)
Thus, it is desirable in SPECT, particularly in nuclear medicine imaging of small organs, such as brain, heart, kidneys, thyroid, that systems, component parts of such systems and methods be developed to improve the reconstruction of transmission data for improved three-dimensional images of the distribution of the radioisotope within the patient.